1. Field of the Invention
The present invention relates to a semiconductor thin film and a process for production thereof. It is applicable to semiconductor devices including polysilicon thin film transistors (for liquid crystal displays), solar cells, and SOI devices.
2. Description of Related Art
A liquid crystal display has thin film transistors (TFTs) as driving devices. The active layer of TFTs is dominated by polysilicon (poly-Si) film rather than amorphous silicon (a-Si) film, because the former permits carriers (electrons in n-channel and holes in p-channel) high mobility and helps realize smaller cells for a fine pitch display. Another advantage of poly-Si film is the capability of low-temperature annealing by laser. This technique, unlike ordinary annealing that needs a high-temperature process (at 1000° C. or above), prevents the substrate from getting hot during annealing and hence permits TFTs to be formed on an inexpensive glass substrate.
The laser annealing process is designed to scan a-Si film formed on a glass substrate with an absorbable laser beam, thereby converting a-Si film into poly-Si film. It is one of the important processes used for the production of low-temperature polysilicon thin film transistors. Laser annealing is accomplished by using a pulsed excimer laser (in the form of linear beam) with a wavelength of 308 nm. The linear beam is required to have a uniform intensity distribution over its long axis. With a uniform intensity distribution, the linear beam offers an advantage of achieving annealing with a less number of scans but suffers a disadvantage of requiring more than 20 overlapping shots. A new idea to eliminate this inconvenience has been proposed. It is based on the principle that crystals grow from a low-temperature region to a high-temperature region under the condition that the intensity distribution of the laser beam is spatially controlled. In this way it is possible to obtain polycrystalline silicon composed of large crystal grains by means of low overlapping shots. One way to spatially control the intensity distribution of laser irradiation is reported in Appl. Phys. Lett. 41 (1982) 346. According to this literature, the object is achieved by forming a patterned antireflection film on an amorphous silicon film, thereby controlling the intensity distribution in the amorphous silicon film. The controlled intensity distribution in turn leads to the controlled temperature distribution which promotes the crystal growth in the lateral direction. This known process is laser annealing with CW laser.
There has been reported a process of irradiation through a phase-shifting mask, in Jpn. J. Appl. Phys. Vol. 38 (1999), pp. L110–112. This process has a disadvantage of giving the phase-shifting pattern which has only one raised and lowered step instead of cyclic steps. As a result, the varying beam intensity gives a non-cyclic pattern on the sample surface and the part with a low intensity remains amorphous. Thus, rendering a sample polycrystalline completely by means of this mask necessitates repeating irradiation on amorphous parts.
There has been disclosed in JP-A No. 82669/2000 a process of forming polycrystalline silicon film for solar cells. This process is premised on coherent light of plane wave and is designed to convert amorphous silicon into polycrystalline silicon by irradiation with a laser beam through a phase-shifting mask having a cyclic pattern. The disadvantage of this process is that crystallization may be incomplete for tiny crystals in the region where the intensity of irradiation is low.
There has also been disclosed in JP-A No. 306859/2000 a process for irradiation through a cyclic slit or a cyclic phase-shifting patterned mask. This process has the same disadvantage as mentioned above. That is, tiny crystals remain in the region where the intensity of irradiation is low.
There has been disclosed in U.S. Pat. No. 6,322,625 and U.S. Pat. No. 6,368,945 and Lamda Physik Corporation's catalog (on Crystal as Optical System, January 2002) a process of irradiating a sample with a laser beam through a mask having a slit pattern which produces a binary pattern intensity distribution. This process, called Sequential Lateral Solidification (SLS) process, consists of repeated crystal growth in the lateral direction in the film by partial laser irradiation and melting to give large crystals. According to this process, the first cycle of irradiation leaves unirradiated regions cyclically and the second cycle of irradiation is carried out with accurate aiming at them
There is disclosed in Jpn. J. Appl. Phys. Vol. 31 (1992) pp. 4545–4549 a process for irradiation with pulsed laser beams, thereby forming laterally grown crystal regions in a semiconductor film on a substrate. Described also therein is the morphology of the laterally grown crystals. The lateral crystal growth is accelerated by the temperature gradient due to device structure.
The prior art technologies mentioned above have disadvantages as follows. The process which involves forming a pattern on an amorphous silicon film is limited in throughput because it needs an additional step for pattern forming. The process for forming large crystal grains by irradiation with laser having a cyclically changing intensity distribution suffers the disadvantage of requiring repeated laser irradiation. This is because the first cycle of irradiation leaves some regions (where intensity is low) amorphous or merely forms tiny crystal grains (100 nm or less) due to insufficient energy and hence such incompletely crystallized regions need the second cycle of irradiation. The repeated irradiation needs an accurate stage as explained in the following. One cycle of laser irradiation gives rise to polycrystalline silicon grains whose particle diameter is 1 μm at most if the thickness of amorphous silicon film is 50 nm and the substrate is kept at room temperature. The second and ensuing cycles of irradiation to completely crystallize the remaining amorphous regions need a stage capable of position control accurate to 1 μm or less. Control in the height direction should also be considered in the case where a mask pattern is transferred to the sample surface through a lens so that a cyclic intensity distribution is produced. The pattern transfer is vulnerable to irregularities on the substrate surface on account of the small focal depth. The small focal depth arises from the fact that the lens to achieve an in-plane resolution of about 1 μm needs a lens with a large numerical aperture (NA). In order to ensure a certain focal depth, it is necessary to reduce the area to be irradiated at one time. This in turn makes it necessary to increase the number of frequencies of scanning. For example, in the case of SLS process that employs a lens having a high resolution of 1 μm or less, it is necessary to repeat scanning 30 times to irradiate the entire surface of a substrate measuring 900 mm in width because one region that is covered by irradiation is 30 mm wide. The foregoing is a hindrance to increasing the throughput of the laser annealing process.